Yes, please be careful missyani! I know that in certain circumstances being common law while you are being sponsored as a dependent child can invalidate your PR.
Dependent children
A son or daughter is dependent when the child:
is under the age of 22 and does not have a spouse or common-law partner;
is over the age of 22 and has been continuously enrolled as a full-time student and depended substantially on the financial support of a parent since before the age of 22;
became a spouse or a common-law partner before the age of 22 and has been continuously enrolled as a full-time student and depended substantially on the financial support of a parent since becoming a spouse or common-law partner, or
is over the age of 22 and depended substantially on the financial support of a parent since before the age of 22 because of a physical or mental condition
I really hope the one in bold above applies to you.
If you didn't declare him, you would have to go back, marry him and somehow prove that you weren't common law when you entered Canada.
Anyways, best wishes for your new baby and I hope you're enjoying Canada!